Each class has three tiers:
1) Basic, which is the starting class, and you don't even need to acquire it.
2) Advanced, such as various Crusaders. To get this, you need to complete a quest. This, of course, gives certain advantages, such as new skills to learn, increased health, and mana.
3) Dark/Light. To get this, you need to complete a quest for the advanced class and choose to side with darkness or light (later in the story, messengers will come from Bracada and Deyia; I think it's clear who represents light and who represents darkness). After that, you can take a quest from the trainers to become, for example, a Black Knight. This again gives new advantages and access to new skills (for example, a sorcerer who becomes a Lich can learn dark magic, as can a Dark Priest).